The Internet makes it easier to break target groups down to the level of 'one to one' marketing. Although this marketing dream is definitively a technical possibility, in practice it appears to be very difficult. People do not like to be approached uninvited via e-mail and the effect of commercial e-mail campaigns is unclear, however well targeted they might be. It is in this context that TPG Post, the Dutch-based (international) postal services company, decided to develop the E-mail Monitor in cooperation with Pro Active International. As the focus of this paper is on consumer behaviour, the authors have targeted the first stage of the E-mail Monitor, which is used to determine what combination of mail and e-mail can best be used to achieve the desired effect.